update the font used for docs.flutter.io (#9337)

* update the font used for docs.flutter.io

* update to use the flutter primary color

* update the header background color

dev/docs/styles.html

@@ -4,12 +4,8 @@
 </style>
 
 <style>
-  header {
-    background-color: #01579b;
-  }
-
   body {
-    font-size: 16px;
+    font-size: 15px;
     line-height: 1.5;
     color: #111;
     background-color: #fdfdfd;
@@ -17,6 +13,10 @@
     -webkit-font-smoothing: auto;
   }
 
+  header {
+    background-color: #01579b;
+  }
+
   h1, h2 {
     font-weight: 300;
   }